Improved vapor phase catalytic oxidation of butane to maleic anhydride

ABSTRACT

Oxidation of butane to maleic anhydride using a vanadium-phosphorus oxide catalyst, in oxidized form, wherein the resulting reduced catalyst is separately regenerated. A recirculating solids reactor can be employed. Less than the stoichiometric amount of oxygen, based on the total amount of butane converted, can be employed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved vapor phase oxidation process forthe preparation of maleic anhydride from n-butane over a catalystcontaining mixed oxides of vanadium and phosphorus.

2. Background

An important route to maleic anhydride is the vapor phase oxidation ofn-butane over a vanadium/phosphorus oxide catalyst. The reaction stepinvolves oxidation of n-butane with air (oxygen) to form maleicanhydride, carbon oxides, water and smaller amounts of partiallyoxidized by-products. Typically, the process is carried out inmultitubular fixed bed reactors. The large exothermal heat and thethermal sensitivity of the n-butane oxidation requires low feedconcentrations, expensive heat-transfer equipment, handling of a largevolume of gas, and good reactor temperature control. Low butaneconcentration is also required to avoid flammability conditions. Themagnitude of some of these problems is reduced when a fluidized bedreactor is used. The temperature can be readily controlled within a fewdegrees because of the intensive catalyst mixing and good heat transfercharacteristics. Higher butane concentrations can be used because thedanger of flammability is greatly reduced by introducing the n-butanedirectly into the reactor rather than premixing it with air (oxygen).However, very high butane concentrations and low oxygen-to-butane ratiosin the reactor may result in over reduction of the catalyst and reducedselectivity to maleic anhydride. Also, significant back-mixing of gasesin the fluidized bed reactor encourages maleic anhydride oxidationreactions which reduce selectivity. In addition, rapidly rising gasbubbles in a fluidized bed reactor result in poorer contact betweengases in the bubbles and the catalyst, making it difficult to achievehigh butane conversion.

A recirculating solids reactor has two reaction zones, in which twoseparate reactions take place, and a catalyst (the solid) whichcirculates between the two reaction zones and takes part in bothreactions. Such reactor systems have found use in catalystic cracking inpetroleum refining and in other reactions. U.S. Pat. No. 4,102,914discloses a process for the preparation of acrylonitrile by passing agaseous mixture comprising propylene, ammonia and molecular oxygen andan ammoxidation catalyst through a reaction zone while controlling thesuperficial linear gas velocity and solids feeds rate to achieve a stateof fast fluidization. In a preferred embodiment the lower part of thereactor serves as a regeneration zone and recycled catalyst from theseparator is contacted with molecular oxygen prior to the addition ofammonia and propylene. U.S. Pat. No. 4,261,899 discloses a process forpreparing phthalic anhydride by oxidizing o-xylene with air (oxygen) ina dilute phase transported bed reactor. Substantially all of theo-xylene is introduced at one end of the reactor while oxygen necessaryfor the reaction and fluidized catalyst are introduced at a plurality ofspaced inlets along the reactor. The catalyst is separated from theproduct gases and recycled. European Patent Office Publication No. 0 034442 discloses a process for preparing unsaturated aldehydes (orunsaturated acids) by passing an unsaturated olefin (or unsaturatedaldehyde) and oxygen into a transport line reactor with a solidoxidation catalyst to achieve substantially plug flow within thereactor. Reaction products are stripped from the catalyst with steam inthe stripper chamber.

The preparation of mixed oxide compositions of vanadium and phosphorusand the use of these as catalysts for the oxidation of hydrocarbons tomaleic anhydride is known in the art. In U.S. Pat. Ser. No. B 330,354and U.S. Pat. No. 4,111,963 the importance of reducing the vanadium usedin a vanadium/phosphorus/oxygen (V/P/O) catalyst to the +4 oxidationstate is described. Preferred is the use of concentrated hydrochloricacid as the reaction medium to bring about this reduction and preferredcatalysts have a phosphorus to vanadium atom ratio of 1:2 to 2:1 and aporosity of at least 35%. In U.S. Pat. No. 3,864,280 the reduction ofthe vanadium in such a catalyst system to an average valence state of3.9 to 4.6 is emphasized; the atomic ratio of phosphorus to vanadium is0.9 to 1.8:1. Isobutyl alcohol is used as a solvent for the catalystpreparation, with the indication that an increase in catalyst surfacearea, over that obtained from use of an aqueous system, is achieved. Theaddition of promoters to the vanadium/phosphorus oxide catalystcompositions used for the oxidation of hydrocarbons to maleic anhydrideis also disclosed in the art. Thus, in U.S. Pat. No. 4,062,873 and U.S.Pat. No. 4,064,070 are disclosed vanadium/phosphorus/silicon catalystcompositions made in an organic medium. In U.S. Pat. No. 4,132,670 andU.S. Pat. No. 4,187,235 are disclosed processes for preparing highsurface area vanadium/phosphorus oxide catalysts. Anhydrous alcohols of1-10 carbon atoms and 1-3 hydroxyl groups are used to reduce thevanadium to a valence of 4.0 to 4.6. Also disclosed are such catalystscontaining up to 0.2 mol, per mol of vanadium, of a transition, alkalior alkaline earth metal, for example, tantalum, titanium, niobium,antimony, bismuth or chromium. In U.S. Pat. No. 4,371,702 is disclosedan improved vanadium/phosphorus oxide catalyst containing the promotercomprising silicon and at least one of indium, antimony and tantalum.Zazhigalov et al., React. Catal. Lett. 24(3-4), 375 (1984), report onmicrocatalytic studies of the catalytic oxidation of butane over a V/P/Ocatalyst and have concluded that maleic anhydride forms mainly due tothe gas phase oxygen. They state that on this catalyst the rate ofmaleic anhydride formation and the process selectivity in the presenceof gas phase oxygen is significantly higher than in its absence andthat, therefore, only a small part of the maleic anhydride can form atthe expense of the catalyst oxygen. C. N. Satterfiled, HeterogeneousCatalysis in Practice, McGraw-Hill Book Company, 1980, page 186,discloses that vanadium and other metal oxides capable of rapid andreversible oxidation-reduction may be used as an oxygen carrier to causea patial oxidation reaction, with the carrier being reoxidized in aseparate reactor. D. J. Hucknall, Selective Oxidation of Hydrocarbons,Academic Press, 1974, page 36, discloses that selective alkene oxidationover a bismuth molybdate catalyst may be carried out either in thepresence or absence of gaseous oxygen.

The following U.S. patents are relevant with respect to a process forthe vapor phase oxidation of n-butane to maleic anhydride over avanadium/phosphorus oxide catalyst, wherein such process the ratio ofn-butane to oxygen is greater than the most commonly employed ratio ofabout 1:10 and the less commonly employed ratio of 1:4. However, in allof these patents the disclosed procedures require more oxygen in thefeed gas than the stoichiometric amount required to convert the totalamount of n-butane converted in the process to maleic anhydride. Nonediscloses isolating the reduced catalyst and reoxidizing it beforecontacting it again with n-butane, none discloses operating with thefeed gas substantially free of oxygen, and none discloses operating withthe feed gas substantially free of oxygen and the reoxidized catalystbeing stripped of oxygen before being contacted again with n-butane.

U.S. Pat. No. 3,899,516 discloses an improved process wherein theoxidant is substantially pure oxygen, rather than oxygen diluted withinert gases, for example, with nitrogen, as in air, and the ratio ofbutane to oxygen is greater than 1:4. Preferably, the ratio is in therange of about 1:1 to 20:1. Mole ratios of 1.6:1 and 3:1 are exemplifiedbut the conversions are 2.2% or less and 1.14%, respectively, so thatthe amount of oxygen present is more than 8 times the stoichiometricamount required to convert the total amount of n-butane converted in theprocess.

U.S. Pat. No. 3,904,652 discloses an improved process comprisingmaintaining, in the reaction zone, an n-butane concentration above 1.7%of the feed, an oxygen concentration of 3-13%, and an inert gasconcentration of 70-95%; converting from 30 to 70% of the n-butane;withdrawing a reactor effluent; separating the maleic anhydride; andrecycling a major portion of the remaining reactor effluent to thereaction zone. In the five runs disclosed in the example, the amount ofoxygen present in at least twice the stoichiometric amount required toconvert the total amount of n-butane converted in the process.

U.S. Pat. No. 4,044,027 discloses, in an example, a process wherein thefeed gas stream contains 9.1% oxygen and 6.2% butane; however, only 25%of the n-butane is converted.

U.S. Pat. Nos. 4,151,116 and 4,244,878 disclose the use of a V/P/Ocatalyst having a promoter which is post-deposited upon its surface. Thepatents further disclose that, typically, the oxidation of butane tomaleic anhydride is carried out by means of air or other molecularoxygen-containing gases such as mixtures of carbon dioxide and oxygen ormixtures of nitrogen or steam with air or oxygen, that air is preferred,and that preferably the concentration of butane in the feed will be 1.0to 1.6 volume % with the oxygen above 10 volume %, and 1.5 to 5 volume %with the oxygen below 0.1 volume %. The examples disclose only the useof a mixture of 1.5 volume % of n-butane in air.

U.S. Pat. No. 4,222,945 discloses a process for oxidizing 10 to 50% of alinear C₄ hydrocarbon by contacting it with oxygen. In the process thehydrocarbon concentration is at least 10 molar % and is higher than theexplosive limit, the oxygen concentration is greater than 13 molar % ofthe total material fed to the reaction, and the concentration of anyinert gas present is less than 70 molar % of the total material fed tothe reaction. The preferred oxygen concentration is 14 to 30 molar %. Inall of the examples, the amount of oxygen present is in excess of thestoichiometric amount required to convert the total amount of n-butaneconverted to maleic anhydride in the process. U.S. Pat. No. 4,317,777discloses a process for oxidizing 10 to 50% of a hydrocarbon whichcomprises at least four linear carbon atoms by contacting it withoxygen, the hydrocarbon concentration being greater than 10 molar % andbeing higher than the explosive limit, the oxygen concentration beinggreater than 13 molar %, and the concentration of inert gas beinggreater than 70 molar % of the total material fed to the reaction. Thepreferred oxygen concentration is 14 to 20 molar %. In the examples, theamount of oxygen present is in excess of the stoichiometric amountrequired to convert the total amount of n-butane converted to maleicanhydride in the process.

U.S. Pat. No. 4,342,699 discloses a process comprising contacting anon-flammable, n-butane-rich feed consisting essentially of about 2 toabout 8 mole % n-butane, about 8 to about 20 mole % molecular oxygen,and a balance of at least one inert gas, and an n-butane oxidationcatalyst, in a heat transfer medium-cooled, tubular reaction zonemaintained under oxidation conditions which are such as to yield arelatively low per pass conversion of n-butane, the catalyst beinggraded along at least a portion of the effective length of the reactionzone so as to provide minimum reactivity nearest the feed end of thereaction zone and maximum reactivity nearest the exit end of thereaction zone, and recycling a portion of the effluent with addition ofmake-up gases comprising n-butane and oxygen.

In all of the examples, the amount of oxygen present is in excess of thestoichiometric amount required to convert the total amount of n-butaneconverted to maleic anhydride in the process.

U.S. Pat. No. 4,352,755 discloses a process for producing a gas streamcontaining at least 2.5% by volume maleic anhydride by oxidizing astraight chain C₄ hydrocarbon with oxygen. The feed stream comprises:the C₄ hydrocarbon (25% to 60%, preferably 40 to 55%, and morepreferably 45 to 50% by volume); oxygen (20 to 45%, preferably 25 to 35%by volume); and, optionally, inert gases (0 to 30%, preferably 5 to 25%,and more preferably 12 to 20% by volume).

An object of this invention is to provide an improved vapor phaseoxidation process for the preparation of maleic anhydride from n-butaneover a catalyst containing mixed oxides of vanadium and phosphorus. Afurther object is to provide a vapor phase oxidation process for thepreparation of maleic anhydride from n-butane over a catalyst containingmixed oxides of vanadium and phosphorus, which process is carried out ina recirculating solids reactor. Recirculating solids, transported bedand riser reactors are well known in the art, as is evident from Kahneyet al., 66th Annual Meeting, American Institute of Chemical Engineers,Philadephia, Pa., November, 1973; U.S. Pat. No. 3,799,868; and Robertsonet al., Chemical Science, 36, 471 (1981). Other objects will becomeapparent hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of a recirculating solids reactorconfiguration in which the reaction zone is comprised of two parts, afluid bed section and a riser section, and the regeneration zone iscomprised of a fluid bed section.

FIG. 2 shows a schematic drawing of a recirculating solids reactorconfiguration in which the reaction zone is comprised of a riser sectionand the regeneration zone is comprised of two parts, a riser section anda fluid bed section.

FIG. 3 shows a schematic drawing of a recirculating solids reactorconfiguration in which the reaction zone is comprised of a fluid bedsection and the regeneration zone is comprised of two parts, a risersection and a fluid bed section.

FIG. 4 shows a plot of conversion of n-butane vs. selectivity forExample 8.

DETAILED DESCRIPTION OF THE INVENTION

The invention herein resides in an improved process for the selectivevapor phase oxidation of n-butane to maleic anhydride using an effectiveamount of a vanadium/phosphorus oxide catalyst, in oxidized form, theimprovement consisting of limiting the amount of oxygen in the feed gasso that it is less than the stoichiometric amount required for the totalamount of n-butane converted in the process, the reduced catalystresulting from the oxidation being separated from the product stream andreoxidized before being contacted again with n-butane. Thestoichiometric amount of oxygen required for the total amount ofn-butane converted in the process is the amount of oxygen required toconvert n-butane to maleic anhydride, that is, 3.5 moles of oxygen foreach mole of n-butane converted. Preferably, the amount of oxygen in thefeed gas is less than about 30% of the stoichiometric amount, that is,less than about one mole of oxygen for each mole of n-butane converted.In view of the extensive art on butane oxidation processes, the totalamount of n-butane converted for a given set of operating conditions canusually be estimated reasonably accurately and, using such conversiondata, the less than stoichiometric amount of oxygen to be included inthe feed can be calculated. If there is any question about the totalamount of n-butane converted, the process can be run under the desiredoperating conditions, the total amount of n-butane converted can bemeasured, and the amount of oxygen in the feed can be adjusted to theless than stoichiometric amount, if necessary. If the process of theinvention is carried out in a batchwise fashion, the n-butane oxidationcan be halted, and the reduced catalyst can be regenerated in place. Itis preferred to carry out the process in a recirculating solids reactorwherein the oxidation of the n-butane is carried out in a reaction zoneand a substantial part of the oxidation of the resultant reducedcatalyst is carried out in a separate regeneration zone.

To increase the selectivity of n-butane to maleic anhydride, it ispreferred to carry out the process of the invention so that the feed gasis substantially free of oxygen, provided the catalyst is not overlyreduced in the process. It is especially preferred to carry out theprocess not only so that the feed gas is substantially free of oxygenbut also so that the oxidized (regenerated) catalyst is stripped of gasphase oxygen before being contacted again with n-butane. This strippingminimizes the amount of gas phase oxygen which may be present when theoxidized catalyst is contacted with the n-butane. In either preferredembodiment, it is especially preferred to carry out the process in arecirculating solids reactor wherein the oxidation of the n-butane iscarried out in a reaction zone and the oxidation of the resultantreduced catalyst is carried out in a separate regeneration zone.

The invention herein also resides in an improved process for theselective vapor phase oxidation of n-butane to maleic anhydride over avanadium/phosphorus oxide catalyst, in oxidized form, the improvementconsisting of carrying out the process in a recirculating solidsreactor. More specifically, such process comprises:

(a) contacting a feed gas containing about 1 mol % to 100 mol %,preferably about 5 mol % to about 20 mol %, n-butane, 0 to 20 mol %oxygen, and the remainder, to 100 mol %, inert gas, with an effectiveamount of vanadium/phosphorus oxide catalyst, in oxidized form,comprised of particles about 20 to about 300 μm in size, in the reactionzone of a recirculating solids reactor, at a temperature of about 300°C. to about 500° C., at a gas residence time in the reaction zone ofabout 0.5 second to about 15 seconds, and at a catalyst residence timein the reaction zone of about 2 seconds to about 5 minutes;

(b) removing the effluent produced in step (a) from the reaction zoneand separating the resultant reduced catalyst from the effluent gases,preferably stripping off any effluent gases from the reduced catalyst,transporting the reduced catalyst to the regeneration zone of therecirculating solids reactor, and recovering maleic anhydride from theeffluent gases;

(c) oxidizing the reduced catalyst in the regeneration zone using anoxygen-containing gas, at a temperature of about 300° C. to about 500°C., at a catalyst residence time in the regeneration zone of about 5seconds to about 5 minutes, and at an oxygen-containing gas residencetime of about 1 second to about 30 seconds; and

(d) recycling the oxidized catalyst from step (c) to the reaction zone.

The invention herein provides significantly higher conversion ofn-butane and higher selectivity to, and yield of, maleic anhydride thancan be obtained with known processes using conventional reactors. Italso provides a highly concentrated product stream, in a reduced totalgas flow, for a desired maleic anhydride production rate. Anotheradvantage of the invention is that good selectivities and conversionscan be achieved at lower temperatures than with conventional knownprocesses.

In conventional fixed or fluidized bed reactors, representative of theart, both the oxidation of n-butane over the vanadium/phosphorus oxidecatalyst and the regeneration (oxidation) of the reducedvamadium/phosphorus oxide catalyst are carried out in the same reactionzone. Under steady-state conditions the rates of these two oxidationsteps must be equal. In order to ensure this equality, sufficient gasphase oxygen must be provided. This may result in reduced selectivity tomaleic anhydride and excessive dilution of the product stream.

In the preferred embodiment employing a recirculating solids reactor,the oxidation of n-butane by the vanadium/phosphorus oxide catalyst, inoxidized form, and the reoxidation of the resultant reducedvanadium/phosphorus oxide catalyst by gas phase oxygen are carried outin separate zones, with the conditions for each zone chosen to optimizethe step being carried out in that zone without any restrictions imposedby the demands of the other step. Some reoxidation of the catalyst maybe carried out in the reaction zone in addition to that carried out inthe regeneration zone, if desired, by introducing some gas phase oxygeninto the reaction zone. As already stated above, if the process iscarried out in a batchwise fashion, the butane oxidation can beinterrupted, and the reduced catalyst can be regenerated in place.

In a further description of the embodiment wherein the oxidation ofn-butane is carried out in the reaction zone of a recirculating solidsreactor, the feed gas is comprised of a mixture of n-butane, oxygen(optional) and inert gas. The n-butane concentration in the feed gas canbe from about 1 mol % to 100 mol %, preferably about 5 to about 20 mol%. Some of the n-butane used in the feed may be provided by theunconverted n-butane which is present in recycled reaction gas. In someinstances, n-butane may be available as the predominant component in amixture of gases including other hydrocarbons. As long as none of theother gases present significantly adversely affects the process, it maybe more convenient to use this n-butane-rich mixture in the feed gas asthe source of n-butane. The oxygen concentration in the feed gas can befrom 0 to about 20 mol %. Air can be used as the source of oxygen. Theremainder of the feed, to 100 mol %, can be any inert gas, such asnitrogen or recycled reaction gas containing mostly carbon monoxide andcarbon dioxide, and possibly unconverted n-butane. Oxidizedvanadium/phosphorus oxide catalyst is introduced into the reaction zone.The vanadium/phosphorus oxide particles are about 20 to about 300 μm insize. The oxidation is carried out at a temperature of about 300° C. to500° C., preferably about 340° C. to about 450° C. The reactor exit gaspressure is typically 0-50 psig. The gas residence time in the reactionzone is about 0.5 second to about 15 seconds, and the catalyst residencetime is about 2 seconds to about 5 minutes. The upper limit of catalystresidence time will, of course, depend on the activity of the catayst.If still active the catalyst can be retained in the reaction zone forlonger than 5 minutes.

The catalyst in the reactor effluent is separated from the effluentgases, and the maleic anhydride product is recovered from the effluentgases, both separations employing conventional techniques and equipment.The separated catalyst is referred to herein as the reduced catalystbecause it is in a lower oxidation state than that of the fresh catalystwhich enters the reaction zone. When appropriate to the embodiment, thereduced catalyst is preferably stripped of any reactor gases and thenrecycled to the regeneration zone. The stripped reactor gases are mixedwith the reactor effluent gases. After maleic anhydride is recoveredfrom the efluent gases of the reaction zone, the remaining gases may bevented or recycled to the reaction zone. Any off-gases or gas from theregeneration zone can be vented after heat recovery.

The reduced vanadium/phosphorus oxide catalyst is reoxidized in theregeneration zone using an oxygen-containing gas, such as air. Theregeneration zone temperature is maintained at about 300° C. to about500° C., the catalyst residence time in the regeneration zone is about 5seconds to, typically, about 5 minutes, and the oxygen-containing gasresidence time is about 1 to about 30 seconds. Total gas flow rate andoxygen concentration must be sufficient to provide the needed oxygen forcatalyst reoxidation to occur within the selected gas and catalystresidence time. The oxidized catalyst is then recycled to the reactionzone.

The required amount of catalyst and the required catalyst circulationrate depend on the extent to which the catalyst oxidation reaction iscarried out in the regeneration zone, as opposed to the reaction zone,the amount of n-butane to be reacted, the amount of mobile (or reactive)oxygen contained by the catalyst, and the reaction zone processconditions that determine the amount of catayst oxygen used per pass.When oxygen concentration in the reaction zone is low, or zero, andsubstantially all of the catalyst oxidation reaction is carried out inthe regeneration zone, a high catalyst circulation rate is required.This rate may be reduced, to the extent that the catalyst oxidationreaction is carried out in the reaction zone.

A recirculating solids reactor can be operated continuously to oxidizen-butane without any gas phase oxygen in the reaction zone. Suchoperation results in higher selectivity to maleic anhydride than can beattained with conventional reactors, provided an adequate catalystcirculation rate is maintained to supply the needed oxidized catalyst.In order to minimize the gas phase oxygen in the reaction zone, gasphase oxygen is stripped from the oxidized catalyst before recycling theregeneration catalyst to the reaction zone.

Alternatively, if a recirculating solids reactor is operated to oxidizen-butane under conditions of temperature, oxygen and n-butane partialpressures and residence time in the reaction zone identical to thoseused in conventional reactors, significantly higher conversion ofn-butane and significantly higher yield of maleic anhydride areobtained.

The high selectivity to maleic anhydride attained in the recirculatingsolids reactor is maintained even if the feed to the reaction zone has avery high butane concentration. The feed can be 100% butane.

Recirculating solids reactors can have many differentreactor/regenerator configurations. For example, the reaction zone ofthe reactor can be comprised of a riser reactor, a fluidized bedreactor, a moving bed reactor, or a combination of both a fluidized bedreactor and a riser reactor. Similarly, the regeneration zone of theregenerator can be comprised of a riser reactor, a fluidized bedreactor, or a combination of both a riser reactor and a fluidized bedreactor. It is to be understood that the invention is not limited to thespecific combinations of reactors recited above. A riser or transportline reactor is characterized by high gas velocities of from about 5ft/s (about 1.5 m/s) to greater than 40 ft/s (12 m/s). Typically, thereactor line is vertically mounted with gas and solids flowing upward inessentially plug flow. The flow can also be downward and the reactorline can be mounted other than vertically. With upward flow of gas andsolids, there can be a significant amount of local backmixing of solids,especially at the lower end of the velocity range. The solidsconcentration in the reaction zone of the reactor can range from,typically, about 1 lb/ft³ (16 kg/m³) to, typically, about 10 lb/ft³ (160kg/m³), depending on the gas velocity, catalyst particle size anddensity, and the catalyst circulation rate. A fluidized bed reactor ischaracterized by extensive solids backmixing and considerable gasbackmixing. The gas velocity ranges from about a few inches per second(5-10 cm/s) to about 3 ft/s (about 1 m/s), and the solids concentrationranges from about 20 lb/ft³ to about 45 lb/ft³ (about 300 kg/m³ to about700 kg/m³) for the vanadium/phosphorus oxide catayst used in thisinvention process, the catalyst having a bulk density of about 50 lb/ft³(about 800 kg/m³). The lower gas velocity and the larger fluidized bedvolume makes the fluidized bed a preferred location to install heatexchangers to remove heat and control temperature. The preferredselection of the reactor and the regenerator configurations depends onthe activity of the catalyst, the attrition resistance of the catalyst,the relative rate of catalytic degradation of maleic anhydride comparedto the production of maleic anhydride, the amount of gas phase oxygenused in the reactor zone, the need for large fluidized bed volume toaccommodate heat exchangers, and the impact of these elements on theoverall economics.

FIG. 1 is a schematic drawing of one of the recirculating solidsreactors used in the examples. The reaction zone is comprised of afluidized bed section 1 and a riser section 2. The feed gas enters 1 andthe oxidation of n-butane takes place in sections 1 and 2. Theseparator-stripper unit 3 separates and strips off the reaction zoneeffluent gases from the reduced catalyst. The maleic anhydride productis recovered from the reactor effluent gases leaving 3. The reducedcatalyst is transported to the regeneration zone which is comprised ofthe fluidized bed section 4. The reduced catayst is oxidized in section4 and the oxidized (regenerated) catalyst is then recycled to thefluidized bed section 1. The alternate/additional feed line 5 can beused to feed additional oxygen to riser section 2. The recirculationsolids reactor of this embodiment can also be operated with just theriser section 2 as the reaction zone. In this mode of operation the feedcan be introduced into the riser section 2 through feed line 5.

FIG. 2 is a schematic drawing of another recirculating solids reactorused in the examples. The reaction zone is comprised of a riser section11. The feed gas enters 11 and the oxidation of n-butane takes place in11. The separator-stripper unit 12 separates and strips off the reactionzone effluent gases from the reduced catayst. The maleic anhydrideproduct is recovered from the reactor effluent gases leaving 12. Thereduced catalyst is transported to the regeneration zone which iscomprises of a riser section 13 and a fluidized bed section 14. Thereduced catalyst is oxidized in this regeneration zone and the oxidized(regenerated) catalyst is then recycled to the riser section 11.

FIG. 3 is a schematic drawing of another recirculating solids reactorused in the examples. The reaction zone is comprised of a fluidized bedsection 21. The feed gas enters 21 and the oxidation of n-butane takesplace in 21. The stripper unit 22 strips off the reaction zone gasesfrom the reduced catalyst before the reduced catalyst is transported tothe regeneration zone. The maleic anhydride product is recovered fromthe reactor effluent. The reduced catalyst is transported to theregeneration zone which is comprised of a riser section 23 and afluidized bed section 24, where it is oxidized. The oxidized(regenerated) catalyst is then recycled to the fluidized bed section 21.

The reaction and regeneration zones can be within a single reactor,although better process control usually is achieved if the two are inseparate units.

The conversion of n-butane in percent is defined as 100 times the numberof mols of n-butane converted, divided by the number of mols of n-butanein the feed. The selectivity to maleic anhydride, in percent is definedas 100 times and the number of mols of n-butane converted to maleicanhydride divided by the total number of mols of n-butane converted. Theyield of maleic anhydride in percent is defined as 100 times the numberof mols of maleic anhydride formed divided by the number of mols ofn-butane in the feed.

As indicated in the Background section of this specification, a numberof V/P/O catalyst preparations and V/P/O promoters are known in the art.The process of this invention is not limited to a particular method ofmaking the catalyst, nor to a particular promoter.

The catalyst used in the examples of this invention were prepared bysubstantially following the procedures disclosed in U.S. Pat. No.4,371,702, particularly the procedures of Examples 1 and 2 thereof. Theuse of the expression "substantially following the procedures" is notintended as an implication that the same catalyst ingredients wereemployed, but rather that the same general techniques were used. Thefollowing general discussion of the catalyst is excerpted from thispatent.

The vanadium/phosphorus oxide (V/P/O) catalyst is made by a processwherein a conventional vanadium compound wherein the vanadium is in the+5 oxidation state, such as in V₂ O₅ or NH₄ VO₃, is initially reduced toa substantial degree to the +4 oxidation state by reaction in either anaqueous or organic liquid medium. In an aqueous medium, the reductantcan comprise a soluble inorganic compound, such as a halide acid, forexample, concentrated hydrochloric acid; a reduced acid of phosphorus,for example, H₃ PO₃ ; or a soluble organic compound, for example,formaldehyde, ethylene glycol, or glycolic, oxalic, citric or tartaricacid. In an organic medium, the preferred medium, the reductant cancomprise an alcohol(s) selected from such species as n-propyl,isopropyl, n-butyl, isobutyl, and benzyl alcohols. The reduction can bebrought about by slurrying the pentavalent vanadium compound in theliquid medium, followed by heating under reflux for the necessary timeto bring about reduction.

Preferably, the V/P/O catalyst includes a promoter which is acombination of selected materials, preferably introduced in both aspecific order and chemical form, following the reduction step in whichthe tetravalent vanadium species is formed. The promoter comprisessilicon and at least one of the variable valent elements selected fromindium, antimony, and tantalum. In such a catalyst the Si/V atom ratiois in the range 0.02-3.0:1.0, and the (In+Sb+Ta)/V atom ratio is in therange 0.005-0.2:1.0, preferably 0.02-0.12:1.0. The P/V atom ratio is inthe range 0.9-1.3:1.0.

In the aqueous system for preparing the catalyst the silicon can beintroduced in the form of a colloidal silica sol, for example, as one ofthe Ludox® colloidal silica compositions commercially available from E.I. duPont de Nemours and Company. In the organic system, for example, analcoholic system, the silicon can be added as an alkyl orthosilicate,for example, tetraethyl orthosilicate. When using an orthosilicate andV₂ O₅ it is preferable to add at least 0.25 mol of orthosilicate per molof V₂ O₅ following substantial reduction of the +5 vanadium to the +4vanadium species.

Although not necessary to the basic performance of such a V/P/Ocatalyst, it is preferred that the indium, antimony and/or tantalum beintroduced into the reaction medium as soluble species. Thus, in theorganic system, they can be added as a cation with an appropriateattendant anion, for example, an acetate, alkoxide, or anhydrous halide.Although the addition of the indium, antimony and/or tantalum compoundcan be carried out during the reduction of the pentavalent vanadiumspecies, it is preferred that this addition takes place subsequent tothe initial charge of silicon compound, for example, an alkylorthosilicate, in order to preclude and/or minimize hydrolysis thereofto a less desirable oxide species of indium, antimony and/or tantalumprior to the ultimate addition of the requisite phosphorus compoundwhich completes the formation of the catalyst precursor. The hydrolyticformation of and/or the primary addition of indium, antimony and/ortantalum as slurried oxides, although still giving operable catalyst,leads to more polyphase products showing macro segregation of theadducts in the final catalyst, particularly at higher levels ofaddition.

Following substantial reduction of the +5 vanadium to the tetravalentspecies and the introduction of the requisite promoter or promoterprecursors the catalyst precursor is formed by the addition of anycommonly used appropriate phosphorus compound, for example, phosphoricacid, in such amount that the P/V atom ratio in the ultimate catalyst isin the range 0.9-1.3:1.0, with continued heating of the resultantmixture under reflux to give the catalyst precursor composition that canbe isolated by filtration, following cooling of the slurry to roomtemperature. This product is subsequently dried in air at 80°-200° C. Itis a crystalline species having an x-ray diffraction pattern (CuKα) withthe following major peaks:

    ______________________________________                                        d-value, Å                                                                              Intensity, I/I.sub.o                                            ______________________________________                                        5.70          100                                                             4.52          43                                                              3.67          29                                                              3.29          37                                                              3.11          17                                                              2.94          54                                                              2.79          14                                                              2.66          16                                                              1.90          11                                                              ______________________________________                                    

This catalyst precursor is then formed into a convenient catalyst shape,for ultimate charge into a reactor, by gently crushing through a 20-meshsieve (U.S. Sieve Series), blending the resultant powder with 1-3% of adie lubricant and pellet binder, such as graphite or Sterotex®, ahydrogenated cottonseed oil, commercially available from Capital CityProducts Co., and tableting to either 1/8" or 3/16" (3.2 or 4.8 mm)diameter pellets.

The pelleted catalyst precursor is fired in a controlled manner in orderboth to generate and activate the catalyst species for use in the vaporphase oxidation of n-butane to maleic anhydride. Typically, unactivatedpellets charged into a 1" (2.54 cm) diameter quartz tube in a verticalfurnace are heated, first, in a low flow of air (about 1-3volumes/volume of catalyst/minute) at 375°-400° C. for 1-6 hours, andthen, in a more rapid gas flow (about 3-6 volumes/volume ofcatalyst/minute) of 1-1.5% n-butane in air (by volume) at 450°-490° C.for about 16-24 hours. The resultant pellets are then ready for use inthe production of maleic anhydride.

In the following examples all temperatures are in degrees Celsius,residence times are in seconds, gas compositions are in mol percent,conversions and selectivities are in percent, and the gas pressure atthe reactor exit was about 1 psig (6.9 kPa).

EXAMPLE 1

A recirculating solids reactor of the type shown in FIG. 1 was used tooxidize n-butane to maleic anhydride. A feed gas consisting of 3 mol %n-butane and 97 mol % nitrogen (that is, free of oxygen) was introducedinto the reactor fluidized bed section 1 and contacted with thevanadium/phosphorus oxide catalyst at a temperature of 438°. Before use,the V/P/O catalyst pellets were crushed and sieved to obtain the desiredcatalyst particle size, about 75 to 250 μm. The total gas residence timein the reaction zone was about 4 seconds.

The regeneration zone was maintained at the same teperature and exitpressure as the reaction zone and a gas stream containing 20 mol %oxygen and 80 mol % helium was introduced into it to oxidize thecatalyst for recycle. The O₂ /He gas residence time was 3 seconds.

Maleic anhydride yields were determined by quenching the product streamwith water and titrating the recovered maleic anhydride in the quenchedwater with 0.1N NaOH solution. Product gas stream samples obtained afterquenching were analyzed for O₂, N₂, CO, CO₂, unconverted n-butane, andlower hydrocarbons. No such hydrocarbon species were detected in any ofthe samples. The conversion of n-butane was 83.3% and the selectivity tomaleic anhydride was 60.8%. The reaction conditions and the resultingdata are summarized in Table I.

The results show that n-butane can be oxidized to maleic anhydride withhigh selectivity at high conversion in the absence of gas phase oxygenin the reaction zone of the recirculating solids reactor. Oxygen for thereaction is supplied by the recirculating oxidized catalyst; the reducedcatalyst is oxidized in a separate regeneration zone.

EXAMPLE 2

Example 1 was repeated, except that the temperature was 376° and thefeed gas into the reaction zone contained 2.1% butane, 8.5% oxygen and89.4% nitrogen. The reaction conditions and resulting data aresummarized in Table I.

The experiment of Example 2 was repeated (Comparative Experiment A),except that the feed gas into the regeneration zone was helium. Thereaction conditions and resulting data are summarized in Table I.

The results show that the selectivities are substantially the same inExample 2 and showing A (outside the invention), presumably because thereaction temperature and the oxygen partial pressure were nearlyidentical. However, Example 2 gave considerably higher conversion thanShowing A because of the recirculation of oxidized catalyst in Example2.

                                      TABLE I                                     __________________________________________________________________________                Reaction Zone  Reg'n Zone                                         React. &    Gas Res.                                                                           Feed Gas  Feed Gas                                                                           Gas Res.                                      Ex. No.                                                                            Regen. Temp.                                                                         Time n-butane                                                                           O.sub.2                                                                         N.sub.2                                                                          O.sub.2                                                                         He Time Conv.                                                                             Sel.                                 __________________________________________________________________________    1    438    4    3    0 97 20                                                                              80 3    83.3                                                                              60.8                                  2.  376    4    2.1  8.5                                                                             89.4                                                                             20                                                                              80 5    59.3                                                                              68.3                                 A.   379    4    2.1  8.5                                                                             89.4                                                                              0                                                                              100                                                                              5    48.1                                                                              67.2                                 __________________________________________________________________________

EXAMPLES 3-8

Examples 3-8 were carried out using the procedure and a recirculatingsolids reactor of the type described in Example 1 to oxidize n-butane tomaleic anhydride. Comparative Experiments B-G (outside the invention)were carried out using a conventional fluidized bed reactor to oxidizen-butane to maleic anhydride.

The V/P/O catalyst was formed into pellets which were calcined in air at390° for 1 hour. The pellets were then crushed and sieved to the desiredsize. The catalyst used was a mixture of 71 parts, by weight, of a V/P/Ocatalyst containing 1% antimony, 1% indium and 2% silicon as promoterand 29 parts, by weight, of a V/P/O catalyst, of similar particle size,containing 4% tantalum and 2% silicon as promoter. This catalyst mixturewas then activated in a fluidized bed reactor using 1.4 mol % n-butanein air at 450° C. for 24 h. The activated catalyst particle size wasabout 75 to about 250 μm.

the reaction conditions and resulting data are summarized in Table II.The results show considerably higher selectivities for Examples 3-8 inwhich the recirculating solids reactor was used with no gas phase oxygenfed to the reaction zone, than for Comparative Experiments B-G in whichthe fluidized bed reactor was used.

                                      TABLE II                                    __________________________________________________________________________    Reaction Zone        Regeneration Zone                                               Gas               Gas                                                  Ex.    Res.                                                                             Feed Gas       Res.                                                                             Feed Gas                                                                           %   %                                        No.                                                                              Temp.                                                                             Time                                                                             n-butane                                                                           O.sub.2                                                                          N.sub.2                                                                          Temp.                                                                             Time                                                                             O.sub.2                                                                          He                                                                              Conv.                                                                             Sel.                                     __________________________________________________________________________    3  357 5  1.2  0  98.8                                                                             355 4  20 80                                                                              46.6                                                                              90.1                                     4  355 5  11.0 0  89.0                                                                             355 4  20 80                                                                               9.0                                                                              90.8                                     5  356 5  5.4  0  94.6                                                                             355 4  20 80                                                                              16.0                                                                              83.2                                     6  350 3.2                                                                              3.5  0  96.5                                                                             350 4  20 80                                                                              16.3                                                                              93.2                                     7  363 5  2.7  0  97.3                                                                             360 4  20 80                                                                              30.2                                                                              85.7                                     8  358 5  1.2  0  98.8                                                                             355 4  20 80                                                                              47.7                                                                              88.2                                     B  356 6.4                                                                              1.3  20.7                                                                             78.0                                                                             n.a.                                                                              n.a.                                                                             n.a. 60.4                                                                              66.0                                     C  363 6.4                                                                              1.0  20.8                                                                             78.2                                                                             n.a.                                                                              n.a.                                                                             n.a. 68.0                                                                              66.7                                     D  357 4.7                                                                              1.0  20.8                                                                             78.2                                                                             n.a.                                                                              n.a.                                                                             n.a. 49.7                                                                              63.2                                     E  369 6.4                                                                              1.3  10.0                                                                             88.7                                                                             n.a.                                                                              n.a.                                                                             n.a. 54.9                                                                              65.3                                     F  355 6.4                                                                              1.3  5.0                                                                              93.7                                                                             n.a.                                                                              n.a.                                                                             n.a. 36.8                                                                              76.0                                     G  354 6.4                                                                              2.8  5.0                                                                              92.2                                                                             n.a.                                                                              n.a.                                                                             n.a. 22.8                                                                              75.6                                     __________________________________________________________________________     n.a. = not applicable                                                    

EXAMPLE 9

Experiments were carried out using three recirculating solids reactorshaving configurations of the type shown in FIGS. 1, 2 and 3,respectively, and, for comparision, using a conventional fluidized bedreactor. The vanadium/phosphorus oxide catalyst used in theseexperiments contained 10 wt % silica (incorporated into the catalystprecursor slurry) to improve attrition resistance of the catalyst. Thecatalyst, after spray drying, was screened and the particles of size 75to 250 μm were calcined in air at 390° for 1 h. The calcined catalystwas then activated by heating at 450° for 16 h in a fluidized bedreactor in an atmosphere consisting of 1.5 mol % n-butane, 12 mol %oxygen and 86.5 mol % nitrogen.

The procedure used to oxidize n-butane in the conventional fluidized bedreactor was substantially the same as that used for ComparativeExperiments B-G. Experiments were carried out at reactor temperatures of375°, 405°, and 420°. In each instance the gas feed consisted of 1.5 mol% n-butane, 12 mol % oxygen and 86.5 mol % nitrogen. The gas residencetime was 7 seconds.

The procedures used to oxidize n-butane in the three recirculatingsolids reactors were similar to the procedure used in Example 1, exceptthat in some of these experiments the reaction zone consisted of afluidized bed reaction section, in some the reaction zone consisted of ariser reaction section, and in other experiments the reaction zoneconsisted of a fluidized bed reaction section and a riser reactionsection. Similarly, in some experiments the regeneration zone consistedof only a fluidized bed reaction section, and in other experiments theregeneration zone consisted of a riser reaction section as well as afluidized bed reaction section. The regeneration zone temperature andpressure in these experiments were the same as the reaction zonetemperature and pressure. The regenerator feed gas consisted of 20 mol %oxygen and 80 mol % helium. The gas residence time in the regenerationzone was about 5 seconds for all experiments. The range of operatingconditions used in the reaction zone during these experiments were:

mol % n-butane in feed gas: 1.0 to 50.0%

mol % oxygen in feed gas: 0 to 8%

balance of feed gas: nitrogen

gas residence time: 0.7 to 7.0 seconds

temperature: 360° and 420°.

FIG. 4 shows the results of these experiments plotted as conversionversus selectivity at two different temperatures used in therecirculating solids reactors. The conversion versus selectivity for thethree experiments carried out in the conventional fluidized bed reactorare also shown in FIG. 4.

The results for the recirculating solids reactors show that conversionincreases as the amount of n-butane in the feed decreases and increaseswith increase in: (a) the mol % of oxygen in the feed, (b) the gasresidence time in the reaction zone, and (c) the reaction zonetemperature. However, at a constant reaction temperature the effect ofthe mol % n-butane in the feed and the gas residence time in thereaction zone on the conversion versus selectivity relationship wassmaller than the scatter in the data. Thus, a change in the mol % ofn-butane in the feed over the range 1 to 50% did not affect theselectivity at any constant conversion achieved by adjusting otherprocess variables. Similarly, the use of three different reaction zoneconfigurations, two different regeneration zone configurations, andseveral different gas residence times did not significantly affect theconversion versus selectivity relationship. About half of theseexperiments were carried out using no gas phase oxygen in the reactionzone and about half were carried out using 8 mol % oxygen in the feedgas. The experiments carried out with no gas phase oxygen in thereaction zone generally showed somewhat higher selectivities at a givenconversion. An increase in reaction temperature resulted in a decreasein selectivity at a given conversion. The selectivities in theexperiments carried out in the conventional fluidized bed reactor weresignificantly lower than those achieved in the recirculating solidsreactors.

These results clearly show that the recirculating solids reactor givessignificantly higher selectivity to maleic anhydride and higherconversion of n-butane, and it eliminates any restrictions on n-butaneconcentration in the feed gas.

EXAMPLES 10-11

A recirculating solids reactor of the type described in Example 1 wasused to oxidize n-butane to maleic anhydride. The procedure used wassubstantially the same as that used in Example 1 and the catalyst wasthe same as that used in Example 9. The two examples were carried outunder substantially identical reaction conditions, except that inExample 11 the oxidized catalyst leaving the regeneration zone wasstripped of oxygen using helium as a stripping gas. This stripping wasdone in order to minimize any oxygen gas from the regeneration zonebeing carried into the reaction zone with the circulating oxidizedcatalyst. The reaction conditions and the resulting data are summarizedin Table III and show that considerably higher selectivity is achievedin Example 11, compared to that of Example 10.

                                      TABLE III                                   __________________________________________________________________________                Reaction Zone  Reg'n. Zone                                        React. &    Gas Res.                                                                           Feed Gas  Feed Gas                                                                            Gas Res.                                     Ex. No.                                                                            Regen. Temp.                                                                         Time n-butane                                                                           O.sub.2                                                                         N.sub.2                                                                          O.sub.2                                                                          He Time Conv.                                                                             Sel.                                __________________________________________________________________________    10   360    3.5  3.0  0 97.0                                                                             20.0                                                                             80.0                                                                             5    51.5                                                                              69.8                                11   360    3.5  3.0  0 97.0                                                                             20.0                                                                             80.0                                                                             5    49.1                                                                              77.0                                __________________________________________________________________________

EXAMPLES 12-14

A recirculating solids reactor of the type used in Example 1 was used tooxidize n-butane in Examples 12, 13, and 14. The procedure used wassubstantially the same as that used in Example 1 and the catayst was thesame as that used in Example 9. The three examples were carried outunder substantially identical reaction conditions, except that differentoxygen concentrations were used in the reaction zone feed gas.

The feed gas compositions for the reaction zone and the regenerationzone, the conditions in the reaction zone and in the regeneration zone,and the results for these examples are given in Table IV. The resultsshow that the selectivity increases as the amount of oxygen in the feedis decreased from 71% of the stoichiometric amount required for thetotal amount of n-butane converted in Example 12, to 28% of thestoichiometric amount required for the total amount of n-butaneconverted in Example 13, to 0% of the stoichiometric amount required forthe total amount of n-butane converted in Example 14.

                                      TABLE IV                                    __________________________________________________________________________                Reaction Zone Reg'n. Zone                                         React. &    Gas Res.                                                                           Feed Gas Feed Gas                                                                            Gas Res.                                      Ex. No.                                                                            Regen. Temp.                                                                         Time n-butane                                                                           O.sub.2                                                                         N.sub.2                                                                         O.sub.2                                                                         He  Time Conv.                                                                             Sel.                                 __________________________________________________________________________    12   360    4.7  12   16                                                                              72                                                                              20                                                                              80  5.0  53.2                                                                              69.5                                 13   360    4.6  12    6                                                                              82                                                                              20                                                                              80  5.0  51.5                                                                              74.8                                 14   360    4.6  12    0                                                                              88                                                                              20                                                                              80  5.0  47.7                                                                              75.2                                 __________________________________________________________________________

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode presently contemplated for carrying out the invention isrepresented by the entire disclosure herein, it being understood thatselection of the best mode will depend on a variety of factors,including yield, selectivity and economics.

I claim:
 1. Improved process for the selective vapor phase oxidation ofn-butane to maleic anhydride over a vanadium/phosphorus oxide catalyst,in oxidized form, the improvement consisting of limiting the amount ofoxygen in the feed gas so that it is less than the stoichiometric amountrequired for the total amount of n-butane converted in the process, thereduced catalyst resulting from the oxidation being separated from theproduct stream and reoxidized before being contacted again withn-butane.
 2. Process of claim 1 carried out in a recirculating solidsreactor, in which the oxidation of the n-butane is carried out in areaction zone, and a substantial part of the reoxidation of theresultant reduced catalyst is carried out in a separate regenerationzone.
 3. Process of claim 1 in which the amount of oxygen in the feedgas is less than about 30% of the stoichiometric amount required for thetotal amount of n-butane converted in the process.
 4. Process of claim 1wherein the feed gas is substantially free of oxygen.
 5. Process ofclaim 4 carried out in a recirculating solids reactor, in which theoxidation of the n-butane is carried out in a reaction zone, and thereoxidation of the resultant reduced catalyst is carried out in aseparate regeneration zone.
 6. Process of claim 4 wherein the reoxidizedcatalyst is stripped of gas phase oxygen before being contacted againwith n-butane.
 7. Process of claim 6 carried out in a recirculatingsolids reactor, in which the oxidation of the n-butane is carried out ina reaction zone, and the reoxidation of the resultant reduced catalystis carried out in a separate regeneration zone.